The present invention relates to an impedance converter used for a condenser microphone, and more specifically, to an impedance converter for a condenser microphone, in which vacuum tubes such as a triode, an amplifier tube and so on are used for converting an impedance.
In an impedance converter used for a condenser microphone, a voice signal input at a high impedance is output from a condenser microphone to an amplifier at a low impedance. Thereby, in a condenser microphone with a small value in a an effective electrostatic capacity, a frequency response at a low frequency band is obtained therefrom. Further, in such a kind of impedance converters, a vacuum tube is mainly used as amplification means for maintaining a suitable voice generated from a condenser microphone.
A conventional impedance converter as described above is shown in a circuit of FIG. 10, which was used for various condenser microphones. The circuit of FIG. 10 includes input terminals 141, 142, a bias circuit 110 provided therein with a gird resistance 111 connected in parallel with each other, a bias resistance 112 and a condenser 113, a vacuum tube for amplification (hereinafter called as an amplifier tube) 120, a load resistance 130, a power terminal 143, an output terminal 144, and a ground terminal 145.
A condenser microphone portion 100 includes a voice signal generating circuit 101 for generating a voice signal, and a condenser 102 which comprises a vibration plate (not shown) in which vibration is caused by sound applied thereto from outside thereof and a fixed electrode (not shown), and connected with input terminals 141 and 142 of an impedance converter. In the voice signal generating circuit 101, a voice voltage is generated, based on an electrostatic capacity corresponding to a change of distance between the vibration plate and the fixed electrode thereby a voice signal is produced. The voice signal produced by the voice signal generating circuit 101 is output to the impedance converter through the input terminal 141 and 142. The condenser 102 comprised of a vibration plate and an electrode has an electrostatic capacity of about 5 to 100 [pF].
The condenser microphone portion 100 is connected with the impedance converter through the input terminals 141 and 142, of which the input terminal 141 is connected with the amplifier tube 120, and of which the input terminal 142 is connected with the ground terminal 145 which is an earth.
In the amplifier tube 120, a vacuum tube for amplification is used for impedance conversion. That is, the amplifier tube 120 comprises a cathode connected with the power terminal 143, a grid connected with the input terminal 141, and a plate connected with the output terminal 144. In such a kind of the amplifier tube 120, a voice signal applied at a high impedance to the grid side is output to the plate side at a low impedance.
A power current Ip flows from the power terminal 143 to the cathode of the amplifier tube 120. When the plate side is an anode, the current flows inward from the grid side. When the plate side is cathode, the current flows from the plate side. Thus, the voice signal input into the impedance converter through the input terminal 141 from a condenser microphone is amplified by the amplifier tube 120 to be output from the plate side. That is, in the amplifier tube 120, when a voice signal is applied, the plate side is at ground so that it is operated as a cathode follower.
The bias circuit 112 is disposed between the grid and the plate in the amplifier tube 120, and comprises a grid resistance 111 and a bias resistance 112 connected in parallel therewith each other. The bias resistance 112 of the bias circuit 110 is connected in series with a load resistance 130, and connected with a grid/ground terminal 145 through the load resistance 130.
The flow of the current Ip to the amplifier tube 120 is lowered in voltage by the bias circuit. In the amplifier tube 120, the plate side is loaded through the bias resistance 112, and the grid side is loaded through the grid resistance 111. An AC component of the power current Ip from the plate side of the amplifier tube 120 is bypassed by the condenser 113.
In the impedance converter with the triode as described above, a voice signal applied to the amplifier tube 120 from the condenser microphone portion 100 is not applied to the grid at the input impedance enough to a bias voltage, since the applied voice signal is affected from the grid resistance 111. As a result, a sufficient signal can not be taken out of the plate side.
When a voice signal is output from the plate of the amplifier tube 120 at a low impedance, a mutual conductance between the cathode and the grid of the amplifier tube 120 has to be increased for the purpose of solving the aforementioned problem. Because the output impedance is 1/mutual conductance and depends on the value of the mutual conductance. Therefore, a vacuum tube with a high mutual conductance value has to be selected as the amplifier tube 120. It is known that when the grid and the plate are spaced at a short distance, the mutual conductance value of the vacuum tube is increased. However, when the spacing between the grid and the plate is at a short distance, free electrons are liberated from the grid so that it tends to cause a flow of much leaky current, and makes the amplifier tube to be operated unstably. Firstly, this problem needs to be solved.
On the other hand, when the resistance value of the grid resistance 111 is increased, the voice signal can be applied to the grid of the amplifier tube 120 at a higher impedance than that of the previous one. However, the grid resistance 111 causes a change in bias voltage of the amplifier tube, since voltages on both ends of the grid resistance are increased. Therefore, it results in making the amplifier tube to be operated unstably.
Further, in the amplifier tube 120 used as a cathode follower, there is a problem in that residual electrons in a heater for heating a cathode are electrostatically coupled and output as a ham noise together with a voice signal.
In view of the above-described problems, an object of the present invention is to provide an impedance converter for a condenser microphone, in which an input voice signal from a condenser microphone portion through an input terminal can be impressed at a sufficient impedance to a grid of an amplifier tube with a high mutual conductance and a sufficient voice signal can be taken out of a plate without a ham noise affected from a cathode.
In order to achieve the aforementioned object, there is provided an impedance converter for a condenser microphone having an input terminal into which a voice voltage signal is input from outside, an amplifier tube which comprises a cathode, a grid and a plate for amplifying said voice voltage to output it to outside therefrom, the amplifier tube being plate-grounded, a power terminal for supplying power to a cathode of said amplifier tube, a bias circuit for applying a bias voltage to a grid of said amplifier tube, a condenser connected with a plate of said amplifier tube, a load resistance connected in series with said condenser, an output terminal for outputting a voice signal amplified by said amplifier tube, and a ground terminal connected in series with said load resistance, characterized in that said bias circuit includes a first diode for loading a flow of a current from a grid of said amplifier tube at a bias voltage, a second diode connected in inversely parallel with said first diode for effecting a rectifying action so that a flow of a current from said first diode flows to the grid, and a bias resistance disposed between said amplifier tube and said load resistance and connected in parallel with said second diode and said condenser to load a flow of current from a plate of said amplifier tube at a bias voltage.
Furthermore, in order to achieve the aforementioned object, there is provided an impedance converter for a condenser microphone having an amplifier tube which comprises an input terminal into which a voice signal is input from outside, a cathode, a grid and a plate for amplifying said voice signal to output it to outside, said amplifier tube with the plate side connected with earth, a power terminal for supplying power to a cathode of said amplifier tube, a bias circuit for applying a bias voltage to a grid of said amplifier tube, a condenser connected with a plate of said amplifier tube, a load resistance connected in series with said condenser, an output terminal for outputting a voice signal amplified by said amplifier tube, and a ground terminal connected in series with said load resistance, characterized in that said bias circuit includes a first diode for loading a flow of the current from a grid of said amplifier tube at a bias voltage, a second diode connected in inversely parallel with said first diode for effecting a rectifying action so that the flow of the current from said first diode flows to the grid, and a third diode disposed between said amplifier tube and said load resistance and connected in parallel with said second diode and said condenser to load a flow of the current from the plate of said amplifier tube at a bias voltage.
In addition, the present invention provides an impedance converter having an amplifier tube with a plate side connected with earth, and a bias circuit for generating a bias voltage applied to a grid of the amplifier tube, said impedance converter further including a resistance for applying a voice voltage to a current flowing from a power terminal when a voice signal is input to a grid of said amplifier tube, said voice voltage being applied to the grid side, a second amplifier tube for applying a voice voltage from a plate to a condenser connected with the amplifier tube with the plate connected with the earth, and a load resistance for applying a voice voltage to a cathode of said second amplifier tube.
Furthermore, the present invention provides an impedance converter having an amplifier tube with the plate-ground, and a bias circuit for generating a bias voltage applied to a grid of the amplifier tube, said impedance converter further including a resistance for applying a voice voltage to a flow of the current from a power terminal when a voice signal is input to a grid of said amplifier tube, said voice voltage being applied to the grid side, a second amplifier tube for applying a voice voltage from a plate to a condenser connected to the amplifier tube with the plate-ground, a load resistance for applying a voice voltage to a cathode of said second amplifier tube, two heaters for heating the cathodes of the amplifier tube with plate-ground and the second amplifier tube, respectively, and an electric double layer condenser for overcoming noises generated from said heaters.
Preferably, in the present invention, an input voltage applied to the amplifier tube is a voice voltage from a condenser microphone.
According to the present invention, a bias voltage is applied to a grid of the amplifier tube through the first diode and the second diode connected in parallel with the first diode. These two diodes make a voltage of the grid to be converged upon a bias voltage. At that time, the two diodes provides an operation with high resistivity, since no current flows into the two diodes. As a result, higher impedance than the prior art can be obtained.
Further, a voltage generated in the third diode connected between the plate and the load resistance is a bias voltage applied to the amplifier tube. Thus, the generated voltage is substantially constant, when the current flows, and thereby a bias voltage can be maintained in a constant level.
Moreover, an electric double layer condenser with a large electrostatic capacity is connected in parallel with a heater so that the an influence of the noises generated from the heater can be eliminated from a cathode of the amplifier.